ActivePython's installer installs logilab-common package into site-package, but not logilab-astng. Even if WebKitPy installs third party libraries by itself and sys.path is setup correctly, the packages in site-package have an priority to be loaded which prevent to import logilab.astng in the thirdpary/autoinstalled directory. The workaround is to install logilab-astng manually globally, but that is not the right solution.
python's cli argument "-S" disables site-package which solve this issue, but need to figure out for the possibility of other new issue.
Actually "-S" doesn't work well because it prevents to load primitive modules.
I confirmed this issue also happens on Mac if logilab-common is installed globally.